#define _CRT_SECURE_NO_WARNINGS 1
#include<iostream>
#include<vector>
using namespace std;
class Solution {
    int tmp[50010];
public:
    int reversePairs(vector<int>& nums) {
        return mergesort(nums, 0, nums.size() - 1);
    }
    int mergesort(vector<int>& nums, int left, int right)
    {
        if (left >= right)
            return 0;
        int ret = 0;
        int mid = (left + right) >> 1;
        ret += mergesort(nums, left, mid);
        ret += mergesort(nums, mid + 1, right);
        int cur1 = left;
        int cur2 = mid + 1;
        int i = left;
        while (cur1 <= mid)
        {
            while (cur2 <= right && nums[cur2] >= nums[cur1] / 2.0)
            {
                cur2++;
            }
            if (cur2 > right)
                break;
            ret += right - cur2 + 1;
            cur1++;
        }
        cur1 = left;
        cur2 = mid + 1;
        while (cur1 <= mid && cur2 <= right)
        {
            tmp[i++] = nums[cur1] <= nums[cur2] ? nums[cur2++] : nums[cur1++];
        }
        while (cur1 <= mid)
        {
            tmp[i++] = nums[cur1++];
        }
        while (cur2 <= right)
        {
            tmp[i++] = nums[cur2++];
        }
        for (int j = left; j <= right; j++)
        {
            nums[j] = tmp[j];
        }
        return ret;
    }
};